1. Field of the Invention
This invention relates to an automatic sound field correcting device for automatically correcting sound field characteristics in an audio system having a plurality of speakers.
2. Description of Related Art
For an audio system having a plurality of speakers to provide a high quality sound field space, it is required to automatically create an appropriate sound field space with much presence. In other words, it is required for the audio system to automatically correct sound field characteristics because it is quite difficult for a listener to appropriately adjust the phase characteristic, the frequency characteristic, the sound pressure level and the like of sound reproduced by a plurality of speakers by manually manipulating the audio system by himself to obtain appropriate sound field space.
An audio system of this kind is disclosed in a Japanese utility model application laid-open under 6-13292. This audio system includes equalizers for receiving audio signals of multiple channels and controlling the frequency characteristics of the audio signals, and a plurality of delay circuits for delaying the audio signals that the equalizers output for the respective channels, and the signals output by the respective delay circuits are supplied to the plurality of speakers. In addition, in order to correct the sound field characteristics, the audio system further includes a pink noise generator, an impulse generator, a selector circuit, a microphone for measuring the reproduced sound reproduced by the speakers, a frequency analyzer and a delay time calculator. The pink noise generated by the pink noise generator is supplied to the equalizers via the selector circuit, and the impulse signal generated by the impulse generator is directly supplied to the speakers via the selector circuit.
When the delay characteristic of the sound field space is to be corrected, the impulse generator directly supplies the impulse signal to the speakers. The microphone collects and measures the impulse sound reproduced by the respective speakers, and the delay time calculator analyzes the measured signal to obtain the propagation delay time of the impulse sound from the position of the speakers to the listening position. Namely, the impulse signals are directly supplied to the respective speakers with delay times, and the delay time calculator obtains the time differences between the time when the respective impulse signals are supplied to the respective speakers to the time when the respective impulse signals reproduced by the respective speakers reach the microphone. Thus, the propagation delay times of the respective impulse sound are measured. Then, by adjusting the delay times of the delay circuits for the respective channels based on the propagation delay times thus measured, the delay characteristics of the sound field space are corrected.
On the other hand, when the frequency characteristics of the sound field space are to be corrected, the pink noise generator supplies the pink noise to the equalizers. Then, the microphone receives and measures the pink noise sound reproduced by the speakers, and the frequency analyzer analyses the frequency characteristics of the respective measured signals. By controlling the frequency characteristics of the equalizers by the feedback control based on the result of the analysis, the frequency characteristics of the sound field space are corrected.
However, if the frequency characteristics of the equalizers are controlled independently for the multiple channels, the phases of the signals of the multiple channels mismatch because the phases of the signals vary when different equalizer coefficients are used for different channels. Normally, when two-channel audio signals are reproduced from a pair of speakers, i.e., a right speaker and a left speaker, if the signals of two channels are in phase with each other (i.e. match), the reproduced sound image locates at a center of the left speaker and the right speaker. Therefore, the listener at the position remote from the both left and right speakers by substantially identical distance feels like the reproduced sound comes from the center of the left and right speakers. However, if the audio signals of the left and right channels are out of phase with each other (i.e., mismatch), the reproduced sound image does not correctly locate at the center of the left and right speakers, and the listener acoustically feels like the sound source is at other position. Therefore, if the audio signals from the left and right speakers are out of phase, the listener feels the reproduced sound coming from unnatural direction and may have strange auditory feeling.
Further, a high-quality type audio system has multiple pairs of left and right speakers positioned forward and backward of the listener, and multi-channel sounds from those speakers are mixed to create the sound field. If the phases of the signals from the pair of the speakers mismatch, correct phantom sound image cannot be created, and the listener feels more strange. This prevents correct sound field reproduction, and consequently damages the presence of the sound field.